1. Field of the Invention
The present invention relates to an image sensor unit and an image reader using the image sensor unit.
2. Description of the Related Art
As an image reader, an apparatus for reading a print pattern of a paper sheet has been proposed.    Patent Document 1: Japanese Laid-open Patent Publication No. 2003-46726
As described in Patent Document 1, the image sensor unit in the image reader (apparatus for reading a print pattern of a paper sheet) has a configuration that emits light having a light emission wavelength in an infrared region in order to identify authenticity of a banknote in identification of a paper sheet.
This is because, printing on surfaces of an authentic banknote provides a region where images to be acquired under visible light and infrared light are different from each other.
In this case, since plastic is more inexpensive in view of cost than optical glass, highly light transmissive plastic, such as acrylic and polycarbonate, is typically used.
Incidentally, for instance, plastics have respective regions with large variation in absorptivity of light of emission wavelengths in an infrared region. In acrylic, the region is in light emission wavelengths in a near infrared region from 780 to 1000 nm. This region is used for determining authenticity of a banknote (see FIG. 3).
On the other hand, in an LED used as a light source, temperature variation causes variation in light emission wavelength (hereinafter, referred to as temperature shift). For instance, in a case of using a typical LED that is inexpensive in cost and emits light of light emission wavelengths in an infrared region and have a peak light emission wavelength in a range from 800 to 1000 nm, variation in ambient temperature causes a temperature shift in light emission wavelength (see FIG. 4).
Thus, there is a possibility that, in a certain ambient temperature, a region with large variation in absorptivity of light of emission wavelengths in the infrared region owing to plastic matches with a region of temperature shift of the light emission wavelength owing to temperature variation, thereby causing instability in illuminance of light of the emission wavelengths in the infrared region. This in turn causes instability in an amount of light reflected from a reading surface of a paper sheet during reading, which affects determination of authenticity.